pretend_controller.rb

class PretendController < ApplicationController
  def create
    render params
  end
end

pretend_controller_spec.rb

require 'rails_helper'

RSpec.describe PretendController, type: :controller do
  it 'is stupid' do
    params_a = {foo: 'bar'}
    post :create, params_a
    expect(response.request.params).to include params_a

    params_b = {foo: 'baro'}
    expect{post :create, params_b}
    expect(response.request.params).to include params_b
  end
end

test output

DEPRECATED: Passing a block to Capybara::server is deprecated, please use Capybara::register_server instead
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}
Forced option passed this will rebuild indexes (drop them and then create them again)
Index created for listings
Index created for people
Index created for projects
6/6 Listings: |================================================= 100 =================================================|
44/44 People: |================================================= 100 =================================================|
3/3 Projects: |================================================= 100 =================================================|

PretendController
  is stupid (FAILED - 1)

Failures:

  1) PretendController is stupid
     Failure/Error: expect(response.request.params).to include params_b
       expected {"foo" => "bar", "controller" => "pretend", "action" => "create"} to include {:foo => "baro"}
       Diff:
       @@ -1,2 +1,4 @@
       -[{:foo=>"baro"}]
       +"action" => "create",
       +"controller" => "pretend",
       +"foo" => "bar",

     # ./spec/controllers/pretend_controller_spec.rb:11:in `block (2 levels) in <top (required)>'

Finished in 2.38 seconds (files took 5.46 seconds to load)
1 example, 1 failure

Failed examples:

rspec ./spec/controllers/pretend_controller_spec.rb:4 # PretendController is stupid